1. Field of the Invention
The present invention relates to a semiconductor laser and more particularly, to a frequency stabilization method of a semiconductor laser, a frequency-stabilized light source and a semiconductor laser module used for the method.
2. Description of the Prior Art
A conventional frequency stabilization method of a semiconductor laser is disclosed in the IEEE Journal of Quantum Electronics, Vol. 28, No. 1, Page 75, Jan. 1992, in which an oscillation frequency of the laser is stabilized at an absorption line frequency of acetylene. FIG. 1 shows a functional block diagram of the conventional frequency stabilization method.
In FIG. 1, the ambient temperature of a semiconductor laser 701 is kept constant by a temperature controller 702. A DC power supply 709 supplies a driving current to the laser 701. The driving current is slightly modulated in frequency by a modulation signal outputted from an oscillator 703, resulting in frequency-modulated output light beams of the laser 701.
One of the output light beams of the laser 701 is emitted from aside face of the laser 701 and used for a given application. The other of the output light beams of the laser 701 is emitted from its opposite side face and goes through an optical lens system 708 to be injected into an acetylene (C.sub.2 H.sub.2) gas cell 704. The light beam transmitted through the acetylene gas cell 704 is detected by a photodetector 705 to produce an electrical output signal. The electrical output signal is inputted into an lock-in amplifier 706 to be detected synchronously with the modulation frequency from the oscillator 703.
The lock-in amplifier 706 produces an electrical output signal proportional to a difference or error between the oscillation frequency of the laser 701 and one of absorption peak frequencies of acetylene in the cell 704. The electrical output signal from the amplifier 706 is fed-back to the driving current through a PID controller 707 in which a Proportional, Integral and Differential (PID) controlling method is employed. Thus, the laser 701 is controlled so that its oscillation frequency is kept to be in accordance with the absorption peak frequency of acetylene.
Due to high stability in the absorption peak frequency, the oscillation frequency of the semiconductor laser 701 can be highly stabilized or locked.
With the conventional frequency stabilization method shown in FIG. 1, to obtain the difference or error between the oscillation frequency and the absorption peak frequency, the gas cell 704 and the lock-in amplifier 706 are required, and the driving current is modulated in frequency to be injected into the laser 701. As a result, there arises disadvantages that large-sized and expensive setups or apparatuses are necessary for carrying out the method and no unmodulated output light beam can be obtained.
In the case of stabilizing the optical output power of the laser 701 during operation, there is another disadvantage that another photodetector is necessary in addition to the photodetector 705.
Further, there is still another disadvantage that stabilizable oscillation frequencies are restricted to the absorption peak frequencies of the gas in the cell 704, so that any or arbitrary oscillation frequencies cannot be selected.
Another conventional frequency stabilization method of a semiconductor laser is disclosed in the Japanese Non-Examined Patent Publication No. 64-74780, Mar. 1989, in which a semiconductor laser temperature is detected from a forward voltage of the laser to keep the temperature constant. FIG. 2 shows a functional block diagram of the conventional frequency stabilization method.
In FIG. 2, a semiconductor laser 803 and a Peltier element 802 which generates and absorbs heat are arranged in a thermostatic bath 803. The laser 803 is driven by a constant current supplied from a DC current source 804. A differential amplifier 805 detects between input terminals or electrodes of the laser 803 its forward voltage drop V.sub.f, and sends an electrical output signal proportional the voltage drop V.sub.f to a temperature controller 806.
In response to the output signal from the amplifier 805, the controller 806 increases or decreases a driving current for the Peltier element 801 to thereby keep the temperature of the laser 803 constant.
In general, the forward voltage drop V.sub.f of the semiconductor laser 803 is expressed as ##EQU1## where I.sub.0 is the forward saturation current, I.sub.f is a driving or exciting current, Tis the absolute temperature of the laser 803, e is the charge of an electron and k is the Boltzmann's constant.
To be seen from the equation (1), the forward voltage drop V.sub.f is inversely proportional to the absolute temperature T. Thus, the absolute temperature T of the laser 803 can be exactly measured from the voltage drop V.sub.f.
The differential amplifier 805 produces an output signal relating to the absolute temperature T from the detected voltage drop V.sub.f and sends the signal to the temperature controller 806. In response to the signal, the controller 806 controls to keep the temperature of the laser 803 constant.
The oscillation frequency of the semiconductor laser 803 is decided by the absolute temperature T and driving current I.sub.f, so that it can be seen that the oscillation frequency is stabilized if both of them are kept constant.
With the another conventional frequency stabilization method shown in FIG. 2, an error tends to arise in detection of the absolute temperature T through the differential amplifier 805 because the laser 803 has an leakage current and a recombination current without luminescence both of which increase with the passage of time, providing fluctuation or deviation in the absolute temperature T and driving current I.sub.f.
As a result, there arises a disadvantage that the output light power and oscillation frequency of the laser 803 deviate from the given values, respectively.
In addition to the above methods, still another conventional frequency stabilization method of a semiconductor laser is disclosed in the Japanese Non-Examined Patent Publication No. 1-238083, Sep. 1989. In the method, similar to the conventional method shown in FIG. 2, absorption peak frequencies of a gas is used.